A fastener is a hardware device that mechanically affixes or joins two or more objects. A threaded fastener is a discrete piece of hardware that has internal or external screw threads. Threaded fasteners are typically used to position objects, to create assemblies by holding together multiple components, and also to facilitate disassembly. The most common types of threaded fasteners are a screw and a nut.
A screw or a bolt is a type of fastener characterized by a helical ridge, known as an external thread wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of an object that has the internal thread formed into the object, or a nut. The screw and nut pairing are generally kept together by a combination of friction between the mating threads, a slight stretch of the screw, and compression of the components in the assembly.
In applications where a fastened assembly is exposed to the elements, corrosion of the fastening hardware is a cause for concern. One type of corrosion that may affect the fastening hardware is galvanic corrosion. Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte.
When two or more different types of metal come into contact in the presence of an electrolyte, a galvanic couple may be generated due to different electrode potentials of the different metals. The electrolyte provides a means for ion migration, whereby metallic ions can move from the anode to the cathode of the galvanic couple. Such a process typically leads to the anodic metal corroding more quickly than it otherwise would, while the corrosion of the cathodic metal is retarded, even to the point of stopping. The presence of electrolyte and a conducting path between the different metal components may cause corrosion where otherwise neither metal component alone would have corroded. Even a single type of metal may corrode galvanically, if the electrolyte varies in composition, thus forming a concentration cell.
Accordingly, design of the fastening system and selection of the component materials in the assembly with an eye toward reducing galvanic corrosion may prove critical to the reliability of the subject assembly.